Process and apparatus for manufacturing polycrystalline silicon, and process for manufacturing silicon wafer for solar cell

ABSTRACT

An object of the present invention is to provide a process and apparatus for the continuous flow production of polycrystalline silicon from metallic silicon or silicon oxide as a raw material and also for the manufacture of a wafer by using it, which process and apparatus permit the mass production at a low cost. The above object can be attained by the manufacture of polycrystalline silicon and a silicon wafer for a solar cell by the following steps: (A) smelting metallic silicon under reduced pressure, carrying out solidification for the removal of the impurity components from the melt, thereby obtaining a first ingot, (B) removing the impurity concentrated portion from the ingot by cutting, (C) re-melting the remaining portion, removing boron and carbon from the melt by oxidizing under an oxidizing atmosphere, and blowing a mixed gas of argon and water to carry out deoxidization, (D) casting the deoxidized melt into a mold, and carried out directional solidification to obtain a second ingot, and (E) removing the impurity concentrated portion of the ingot obtained by directional solidification by cutting.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process and apparatus for manufacturingpolycrystalline silicon and a process for manufacturing a silicon waferfor a solar cell. In particular, this invention pertains to a techniquewhich employs metallic silicon or silicon oxide as a starting materialand permits the continuous flow production from polycrystalline siliconto an end product, that is, a polycrystalline silicon wafer for a solarcell.

2. Description of the Related Art

Studies on solar cells have been made for many years. Recently, thosehaving a photoelectric transfer efficiency of even about 13 to 15% undersun light on the ground have appeared and they are now beingindustrialized for various applications. In our country, however, solarcells are not so popular as an energy source for domestic electricpower, automobiles, ships or machine tools, because a technique tomass-produce a silicon wafer at a low cost, which is necessary for themanufacture of solar cells, has not yet been established.

At present, for the manufacture of a silicon wafer for a solar cell, ahigh-purity silicon, which is in the mass form and conforms to thespecification of a semiconductor, is once manufactured through achemical process by using as a starting material a low purity metallicsilicon (99.5 wt. % Si). Then, high-purity silicon in the mass form isre-melted and is adjusted to have a chemical composition suited to asolar cell by a metallurgical process. The resulting molten silicon isformed into an ingot by the pulling method or directional solidificationmethod, followed by slicing into thin plates. Described specifically, asshown in FIG. 5, metallic silicon is first reacted with hydrochloricacid and formed into a trichlorosilane gas. After the gas so obtained isfractionated to remove the impurity elements, the residue is reactedwith a hydrogen gas, whereby high-purity silicon is precipitated fromthe gas by the so-called CVD (Chemical Vapor Deposition) method. Thehigh-purity silicon therefore becomes only an aggregate of silicongrains owing to the weak bonding power between crystal grains. The boroncontained in the high-purity silicon forming the aggregate is reducedeven in the order of 0.001 ppm and does not reach the concentrationnecessary for satisfying the specific resistivity of 0.5 to 1.5 ohm.cmwhich is the specification for P-type semiconductor wafer. In order touse the above high-purity silicon for a solar cell, it is indispensableto adjust the specific resistivity and to control the crystallinity ofsingle crystals or crystal grains so as to have a particle size notsmaller than several mm and have a grain boundary so as not to exertadverse effects on the photoelectric transfer efficiency. The abovesilicon cannot be formed into a wafer without further treatment. Asshown in the right hand of FIG. 5, it becomes necessary to form a waferafter re-melting the high-purity silicon mass, adjusting the componentsof the melt (by the addition of boron) and forming into an ingot(pulling method for single crystals, while directional solidificationfor polycrystals).

The above-described manufacturing method is however accompanied with thedrawbacks that it requires much labor to re-adjust (mainly, by theaddition of boron) the components of a silicon ingot, which has a purityintentionally heightened to be suitable for semiconductor, to besuitable for solar cells or to purify the ingot; its yield is inferior;it additionally requires equipment and energy for re-melting; andtherefore, it is expensive. As described above, the solar cellsavailable now are therefore expensive, which prevents them from beingpopularly used. The purity heightening of metallic silicon by a chemicalprocess is also accompanied by the generation of a large amount ofpollutants such as silane and chloride, which prevents mass-production.According to the above described technique, the manufacturing processtends to be studied, divided into steps such as increasing the purity ofmetallic silicon, or using the solidification technique, which ispresumed to be influenced by the above-described manufacturing method.

For example, Japanese Published Unexamined Patent Application No. HEI5-139713 discloses a process in which silicon having a low boron contentis obtained by maintaining molten silicon in a container composed ofsilica or composed mainly of silica, and injecting a plasma gas jet flowof an inert gas to the surface of the molten silicon, while blowing aninert gas upwardly from the bottom of the container. Japanese PublishedUnexamined Patent Application No. HEI 7-17704 discloses a processpermitting the efficient removal of boron by forming 1.5 to 15 kg ofSiO₂ per kg silicon in advance on the surface of metallic siliconpowders upon melting metallic silicon through an electron beam.Concerning solidification technique, Japanese Published UnexaminedPatent Application No. SHO 61-141612 proposes a technique to prevent,upon casting molten silicon into a mold, precipitation of inclusion in asilicon ingot by turning the mold. In addition, the present applicantsthemselves are now proposing a method for purifying molten metallicsilicon by directional solidification in Japanese Patent Application HEI7-29500 (filed on Feb. 17, 1995).

It is impossible to say that there does not exist a technique tomanufacture solar cell silicon directly from metallic silicon. Forexample, Japanese Published Unexamined Patent Application No. SHO62-252393 discloses a process in which a starting material silicon,which is once used as a semiconductor but disposed as an electronindustry waste, is subjected to zone melting by plasma jet generated bya mixed gas of argon, hydrogen and oxygen. This process aims principallyat the use of an industrial waste so that it does not become amainly-employed technique suited for mass production of a silicon wafer.In addition, although silicon is used as a raw material, its purity hasbeen once increased so that the process is only a variation of theabove-described cumbersome manufacturing process. Japanese PublishedUnexamined Patent Application No. SHO 63-218506 discloses a process formanufacturing, by plasma melting, silicon in the mass form for solarcells or electronics from metallic silicon in the form of powders,granules or polished dusts. This method is based on the principle of thezone melting method using the same plasma as that disclosed in the aboveJapanese Published Unexamined Patent Application No. SHO 62-252393 andis accompanied with the drawback that mass production cannot be carriedout in spite of large electricity consumption. According to Examples ofthe above official gazette, only a silicon rod of 50 g or so is obtainedon a laboratory scale and it does not include a description ofincreasing the size of the silicon wafer for a solar cell to a practicalsize.

SUMMARY OF THE INVENTION

With the forgoing in view, an object of the present invention is toprovide a process and apparatus for mass-producing, at a low cost incontinuous flow production, polycrystalline silicon by using metallicsilicon or silicon oxide as a starting raw material, and a wafermanufactured using the process.

With a view to attaining the above object, the inventors of the presentinvention have carried out an extensive investigation, paying attentionto obtaining the maximum economic effects without using a chemicalprocess but only a metallurgical process, leading to the completion ofthe present invention.

In a first aspect of the present invention, there is thus provided aprocess for manufacturing polycrystalline silicon from metallic silicon,which comprises the following steps:

A: melting metallic silicon under vacuum to remove the phosphoruscontained therein by evaporation, and then carrying out solidificationof the residue for the removal of the impurity elements from the moltensilicon (which may hereinafter be called "melt"), thereby obtaining afirst ingot;

B: removing the impurity concentrated portion of the first ingot bycutting;

C: re-melting the remaining portion, removing boron and carbon from themelt by oxidizing under an oxidizing atmosphere, and in succession,blowing an argon gas or a mixed gas of argon and hydrogen into the meltfor deoxidization.

D: casting the deoxidized melt in a mold, followed by directionalsolidification to obtain a second ingot; and

E: removing the impurity concentrated portion of the second ingot bycutting.

In a further aspect of the present invention, there is also provided aprocess for the preparation of polycrystalline silicon, wherein in theabove-described process, said metallic silicon is obtained by reductivesmelting of silicon oxide.

In a still further aspect of the present invention, there is alsoprovided a process for the preparation of polycrystalline silicon, whichcomprises transferring said metallic silicon under molten state, whichhas been obtained by smelting of silicon oxide in the above-describedprocess, into a crucible, removing boron and carbon from it by oxidizingunder an oxidizing atmosphere, and carrying out solidification, followedby the above-described step B, melting under vacuum, and theabove-described steps D and E.

In a still further aspect of the present invention, there is alsoprovided a process for the preparation of polycrystalline silicon, whichcomprises forming the above-described oxidizing atmosphere from an H₂ O,CO₂ or O₂ gas in an amount small enough so that the whole interfacebetween the melt and the gas will not be covered with silicon oxide,removing silicon oxide formed on the surface of the melt by locallyheating by plasma arc, or blowing an H₂ O, CO₂ or O₂ gas into the meltinstead of placing the melt under the above-described oxidizingatmosphere.

In a still further aspect of the present invention, there is alsoprovided a process for the preparation of polycrystalline silicon, whichcomprises using SiO₂ or Si₃ N₄ as a mold releasing agent, setting asolidification interface moving rate at 5 mm/min or less, saidsolidification being carried out for the removal of impurities, settinga solidification interface moving rate at 2 mm/min or less fordirectional solidification, or cutting the ingot at a height at least70% above the bottom of the ingot.

In a still further aspect of the present invention, there is alsoprovided a process for the preparation of polycrystalline silicon whichcomprises setting a phosphorus concentration of the melt at 0.3 ppm orless and a boron concentration at 0.6 ppm or less or a carbonconcentration at 10 ppm or less.

The present invention also relates to an apparatus for manufacturingpolycrystalline silicon. In a still further aspect of the presentinvention, there is also provided an apparatus for manufacturingpolycrystalline silicon, which comprises heating means for melting orheating metallic silicon, a retaining container for retaining moltenmetallic silicon, a first mold in which the melt is cast from theretaining container, a vacuum chamber for removing phosphorus byevaporation, said chamber surrounding the retaining container and thefirst mold, re-melting means for re-melting or heating a portion of theingot from the first mold, a smelting container for retaining there-melt, a nozzle for blowing or spraying an oxidizing gas, hydrogen gasor a mixed gas of hydrogen and argon to the re-melt in the smeltingcontainer and a second mold for forming the deoxidized re-melt into acast ingot.

In a still further aspect of the present invention, there is alsoprovided an apparatus for manufacturing polycrystalline silicon, whereinthe degree of vacuum in the above-described vacuum chamber is set at10-3 torr or higher, the retaining container is a water-cooling jacketmade of copper or a graphite crucible; and the smelting container is acrucible made of SiO₂, an SiO₂ stamped crucible or an SiO₂ linedcrucible.

In a still further aspect of the present invention, there is alsoprovided an apparatus for manufacturing polycrystalline, wherein theabove-described heating means is an electron gun; or the above-describedre-melting means is a plasma torch or a DC arc source.

In a still further aspect of the present invention, there is alsoprovided an apparatus for the preparation of polycrystalline silicon,wherein the above-described first and second molds have side wallsformed of a heat insulating material and have a bottom formed of a watercooling jacket; and a heating source for heating the cast melt isdisposed above the molds; or a W/H ratio, that is, the ratio of thediameter W to the height H of said mold is set at greater than 0.5.

In a still further and essential aspect of the present invention, thereis thus provided a process for the manufacture of a silicon wafer for asolar cell, which comprises slicing an ingot of polycrystalline silicon,which has been obtained by any one of the above-described processes, toa thickness of 100 to 450 μm.

According to the present invention, polycrystalline silicon or a siliconwafer for a solar cell is manufactured by any one of the above-describedmethods or apparatuses so that the component adjustment of high-puritysilicon, which is indispensable in the conventional method, is notrequired. The present invention also makes it possible to reduce theunnecessary consumption of energy. Since not a chemical process which ischaracterized by the generation of a large amount of pollutants but onlya metallurgical process is adopted, the present invention makes itpossible to enlarge the production equipment. As a result, a siliconwafer for a solar cell having excellent photoelectric transferefficiency can be provided at a cost by far lower than the conventionalone. Furthermore, polycrystalline silicon obtained by the enforcement ofthe present invention can be used effectively not only for themanufacture of a wafer but also for the use as a raw material for ironmanufacture or the like.

As described above, the present invention makes it possible to avoid theconsumption of unnecessary energy and enlarge the manufacturingequipment, thereby mass-producing polycrystalline silicon or polysiliconwafer for a solar cell having relatively good purity. As a result, apolycrystalline silicon wafer for a solar cell which has a photoelectrictransfer efficiency on the ground on the same level with that obtainedin the conventional method can be obtained at a markedly low cost, fromwhich the wide diffusion of solar cells are much expected.Polycrystalline silicon can be used effectively as a raw material foriron manufacture as well as that for a wafer.

According to the present invention, high-purity polycrystalline siliconand a silicon wafer for a solar cell can be manufactured through acontinuous flow production based on only a metallurgical process.Accordingly, the equipment can be enlarged freely and unnecessary energycan be omitted. The present invention is therefore very useful for themanufacture of a silicon wafer for a solar cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating one embodiment of a manufacturingprocess of polycrystalline silicon and a silicon wafer for a solar cellaccording to the present invention;

FIG. 2 is a flow chart illustrating another embodiment of themanufacturing process of polycrystalline silicon and a silicon wafer fora solar cell according to the present invention;

FIG. 3 is a schematic view illustrating an apparatus embodying themanufacturing process of polycrystalline silicon and a silicon wafer fora solar cell according to the present invention;

FIG. 4 illustrates another apparatus embodying the manufacturing processof polycrystalline silicon and a silicon wafer for a solar cellaccording to the present invention; and

FIG. 5 is a flow chart illustrating the conventional process formanufacturing a silicon wafer for a solar cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, one embodiment of the manufacturing process ofpolycrystalline silicon and a silicon wafer for a solar cell accordingto the present invention is shown together in one flow chart(manufacture of the wafer is shown, enclosed with a dotted line).

First, metallic silicon having a relatively low purity (99.5 wt. % Si)is charged in a retaining container made of graphite or a water-coolingretaining container made of copper and then melted under vacuum. At thistime, heating may be conducted making use of the methods known to datesuch as gas heating or electric heating, with heating by an electron gunbeing most preferred. Here, the metallic silicon so melted is maintainedfor a predetermined time (for example, 30 to 60 minutes) in the aboveretaining container at a temperature not lower than 1450° C. but nothigher than 1900° C., whereby phosphorus and aluminum, among impurityelements contained in the melt, are removed by evaporation (vacuumsmelting). It is preferred that the phosphorus concentration in the meltis 0.3 ppm or less. Then, in order to remove the impurity elements suchas Fe, Al, Ti and Ca to be 100 ppm or less, the melt is cast into afirst cast and is cooled upwardly from the bottom so that the movingrate of solidification interface will be 5 mm/min. As a result, an ingotin which the melt having concentrated impurity elements has beensolidified last is obtained.

In succession, the upper 30% portion of the ingot having theconcentrated impurity elements therein is removed by cutting. Theremaining portion of the ingot is charged in a melt furnace equippedwith, for example, a plasma arc, whereby the ingot is re-melted. Also inthis case, the heating means is not limited to the plasma arc. The meltis heated to a temperature not lower than 1450° C. and at the same timeis reacted with an oxidizing gas atmosphere, whereby boron and carbonare removed from the melt as oxides (oxidative smelting). Afteroxidative smelting, an argon gas or a mixed gas of argon and hydrogen isblown into the melt for a predetermined time. As a result, oxygen in themelt is deoxidized to the level not higher than 10 ppm. Incidentally,the above-described oxidative smelting may be carried out either in avacuum chamber or in the air. The deoxidized melt is then cast into asecond mold coated with a mold releasing agent, followed by directionalsolidification, whereby a final ingot is obtained. Impurity elementsexist in the concentrated form in the upper portion of the ingot so thatthe portion (generally, 20% or so) is removed by cutting and theremaining portion is provided as a product of polycrystalline silicon.

Polycrystalline silicon is prepared as described above. It is onlynecessary to slice the above-described remaining portion by a multi-wiresaw into thin plates of 100 to 450 μm thickness.

Metallic silicon, which is a starting material, is generally availableby reductive smelting of silicon oxide so that the use of silicon oxideas a starting material is also added to the present invention. Any knownmethods can be employed to smelt silicon oxide into that having a purityon the same level with that of the metallic silicon used in the firststep of the present invention. For example, silicon oxide is melted andreduced by using a carboneous material as a reducing agent. In thepresent invention, considered is a method of removing the components,which are not necessary for polycrystalline silicon or a silicon waferfor a solar cell, in advance upon obtaining metallic silicon fromsilicon oxide. It is a method as shown in the flow chart of FIG. 2,wherein metallic silicon which has been obtained from silicon oxide, hasa relatively low purity and is under molten state is charged in asmelting container (for example, crucible) and so-called preliminarysmelting is effected. Described specifically, an oxidizing gas (H₂ O,CO₂ or the like) is blown into the melt in the crucible, boron andcarbon are removed as oxides and then, the residue is solidified. Theingot so obtained is melted in the above-described vacuum chamber,phosphorus is removed from the melt by vacuum smelting and the residueis subjected to directional solidification, whereby an ingot ofpolycrystalline silicon is obtained. It is only necessary to slice theingot into thin plates as described above to obtain a wafer. Thisprocess has a merit in that the above-described steps of "boron andcarbon removal" and "solidification for the removal of impurities" ofthe present invention can be omitted by changing a part of ordinarymetallic silicon preparation operations. As a result, this process makesit possible to omit some of the apparatuses and brings about effects forreducing energy consumption, whereby polycrystalline silicon and asilicon wafer for a solar cell on the same level with those obtained bythe above-described process of the present invention are available at alower cost. In particular, if boron and carbon removal is conducted bythose who prepare metallic silicon, operations subsequent to it can becarried out more easily by the manufacturer of polycrystalline siliconor wafer.

Incidentally, the reason for setting the moving rate of thesolidification interface at 5 mm/min or lower in the case of the firstmold and at 2 mm/min in the case of the second mold is because movingrates higher than the above disturb sufficient concentration of impuritymetal elements in the upper part of the ingot. The reason for cuttingthe ingot at a height not lower than 70% from the bottom of the ingot isbecause the target composition as polycrystalline silicon can beattained at the remaining lower portion. In the present invention, thedegree of vacuum in the vacuum chamber is set at 10-3 torr or higherbecause it is suited for phosphorus removal by evaporation judging fromthe vapor pressure of phosphorus in metallic silicon.

In the present invention, the phosphorus concentration of the melt isset at 0.3 ppm or lower in order to secure stable operation of solarcells, while the boron concentration of the melt is set at 0.6 ppm orlower in order to obtain polycrystalline silicon suited for a P-typesemiconductor wafer. The carbon concentration set at 10 ppm or lowermakes it possible to suppress the precipitation of SiC in siliconcrystals, thereby preventing the lowering in the photoelectric transferefficiency.

Furthermore, in the present invention, a copper-made water-coolingjacket or a graphite crucible is employed as the above-describedretaining container upon melting of metallic silicon and an SiO₂crucible or SiO₂ stamped or lined crucible is used as theabove-described smelting container, because silicon tends to react withother substances and when a crucible made of another substance is used,component elements of the substance is mixed in silicon. Incidentally,when boron is removed upon preparation of metallic silicon, inexpensiveAl₂ O₃, MgO, graphite or the like can be employed for the lining of therefractory, because if impurities are mixed in, they can be removed atthe subsequent step. The mold releasing agent of the mold used forsolidification is specified to SiO₂ or Si₃ N₄ because of the samereason. Since the molten silicon expands by 10% in volume whensolidified, the mold releasing agent is necessary for preventing thestress from remaining on the ingot.

In addition, an apparatus according to the present invention isconstructed so that as shown in FIG. 3, the melt 2 of metallic silicon 1flows to the subsequent stage almost continuously except at the time ofsolidification. This structure makes it possible to carry outpreparation smoothly and to shorten the operation time, leading to thereduction in the manufacturing cost. Besides, since the apparatuses usedin the present invention are operated based on only the metallurgicalprocess, they can be enlarged considerably and are free from generationof pollutants. Cost reduction by mass production can also be expected.

The oxidizing atmosphere for the removal of boron and carbon from themelt 2 is not required to have high acidifying power. Preferred as theoxidizing gas is H₂ O or CO₂. When acidifying power is high, an SiO₂film is formed on the surface of the melt, which hinders the removal ofboron and CO₂. In such a case, injection of arc from a plasma torch 4 orDC arc source is necessary for the removal of such a film. Theabove-described oxidizing gas may be blown directly into the melt. Thematerial of a nozzle 5 from which the oxidizing gas is blown is limitedto graphite or SiO₂, because other materials contaminate the melt 2.Incidentally, as a cutting machine (not illustrated) for cutting theingot 6a released from the second mold 9 into thin plates, a knownmulti-wire saw or multi-blade saw can be used without problems. Thereason why the thickness of the thin plate is set at 100 to 450 μm isbecause the plate is too weak at the thickness less than 100 μm, whileit has lowered photoelectric transfer efficiency at the thicknessexceeding 450 μm.

In the apparatus according to the present invention, a particularconsideration is taken for the structure of the mold 9 in whichsolidification is carried out. Described specifically, as shown in FIG.3, the mold is shaped into a so-called washball having a diameterW:height H ratio of 0.5 or greater. In addition, it is constructed tohave a heat insulating material 11 as a side wall, a water-cooled jacket10 as a bottom and a heating source 8 disposed in the upper part of themold so that the moving rate of the solidification interface can beregulated.

In the present invention, it is also possible to carry out thesolidification operations (solidification--re-melting) in the first moldand second mold in repetition. Alternatively, after a plurality of moldsare provided and the above-described retaining container or smeltingcontainer is enlarged, the melt may be poured from the enlargedcontainer in portions to the plural molds. Moreover, it is not necessaryto effect the steps A, B, C, D and E in this order except that the stepsD and E come last.

EXAMPLE 1

As shown in FIG. 3, an electron gun 3 of 300 KW in output was installedon the upper part of a vacuum chamber 18. Metallic silicon 1 was fed toa retaining container 19 (which is also called a melting furnace) madeof graphite at 10 kg/hour and was melted using heating means 7. At thistime, the degree of vacuum in the vacuum chamber 18 was 10⁻⁵ torr. Fromthe melt 2, a portion of phosphorus and aluminum elements wereevaporated and removed 17. The remaining melt 2 was then cast into awater-cooling type copper-made mold 9. While the surface of the melt wasexposed to electron beam 3 to maintain the molten state, the melt wassolidified from the bottom at a solidification interface moving rate of1 mm/min, whereby 50 kg of an ingot 6a were obtained. The upper 20%portion of the ingot 6a (the portion A) was removed by cutting to obtainan ingot having a chemical composition as shown in Table 1.

                  TABLE 1    ______________________________________    (Unit: ppm)    B         P      Fe     Al   Ti   La    C     O    ______________________________________    Metallic            7     23     980  860  180  950   ˜5000                                                    --    silicon    Ingot after            7     <0.1   10   8.5  2    10    35    --    crude    purification    Wafer   0.1   <0.1   <0.1 <0.1 <0.1 <0.1  3.5   5.7    ______________________________________

The remaining portion of the ingot 6a was then melted in a silicacrucible (smelting container) 16 above which a plasma torch 4 of 100 KWin output was disposed. The melt was kept at a temperature of 1600° C.and a mixed gas 21 of argon and water vapor, said gas containing 15 vol.% of water vapor, was sprayed to the surface of the melt. At this time,a sample was taken from the melt 2 and its specific resistivity wasmeasured. About two hours later, the specific resistivity became 1ohm.cm so that the mixed gas 21 was changed to an argon gas anddeoxidization was effected for 30 minutes. The melt was then poured intoa second mold which was made of graphite and coated with Si₃ N₄ as amold release agent and was solidified by cooling upwardly from thebottom under an argon gas atmosphere, whereby an ingot was obtained. Atthis time, a graphite heater 8 was disposed in the upper part of themold 9 by which the surface of the melt was heated. As a result, themoving rate of the solidification interface was 0.7 mm/min.

After the completion of the solidification, the upper 30% of the ingot6b so obtained (the portion B) was removed by cutting and the remainingportion of the ingot was provided as a product of polycrystallinesilicon. The product so obtained was sliced into thin plates having athickness of 350 μm, by a multi-wire saw, whereby 300 silicon wafers 20for solar cells, each wafer having a size of 15 cm×15 cm, weremanufactured. These wafers each had a specific resistivity of 1.2ohm.cm, had a minority carrier whose life time was 12 μsec and, had aphotoelectric transfer efficiency of 13.8%. Its chemical composition isas shown in Table 1.

EXAMPLE 2

In a similar manner to Example 1, an ingot 6a was obtained from thefirst mold. The upper 70% portion of the ingot was melted in a silicacrucible (smelting container) 16 above which a plasma torch 4 of 100 KWin output was disposed. Into the melt 2 maintained at 1600° C., a mixedgas 21 of argon and water vapor, said gas containing 15 vol. % of watervapor, was blown at a rate of 10 liter/min through a porous plug 15disposed at the bottom of the crucible 16, whereby boron and carbon wereremoved from the melt. The residue was subjected to deoxidization,directional solidification and removal by cutting, whereby a product ofpolycrystalline silicon was obtained. The product was sliced in asimilar manner to Example 1, whereby silicon wafers for solar cells weremanufactured.

The size, number and performance of the wafer so obtained were much thesame with those of the wafer obtained in Example 1.

EXAMPLE 3

Using silicon oxide as a starting material, an arc electric furnace 12as shown in FIG. 4 and a carbonaceous reducing agent, melting andreduction were carried out, whereby molten metallic silicon having achemical composition as shown in Table 2 was manufacture. In a crucible14 equipped with a porous plug 15 at the bottom thereof and lined with asiliceous refractory, 50 kg of the metallic silicon 1 were charged.Then, a mixed gas of argon and water vapor, said gas containing 20 vol %of water vapor, was blown into the melt for 30 minutes through theporous plug 15. The remaining melt 2 was heated to 1650° C. by theoxidizing heat of silicon and boron- and carbon-removal reactionoccurred. The melt 2 was cast into a first mold which had an SiC-madeheater disposed in the upper part of the mold and had a bottom coolingsystem, and was solidified by cooling at a moving rate of thesolidification surface at 1.5 mm/min. The lower 80% portion of the ingotso obtained was melted in succession in the retaining container disposedin the above-described vacuum chamber, followed by dephosphorization anddeoxidization. The resulting melt was poured into the second mold,whereby directional solidification was effected. The upper 30% portionof the ingot 6 so obtained was removed by cutting and the remainingportion was provided as a product of polycrystalline silicon. Theproduct was sliced by a multi-blade saw into thin plates of the abovesize, whereby 300 polycrystalline silicon wafers for solar cells wereobtained. The wafers each had a specific resistivity of 0.9 ohm.cm, hada minority carrier whose life time was 10 μsec and had a photoelectrictransfer efficiency of 13.5%. It had a chemical composition as shown inTable 2.

                  TABLE 2    ______________________________________    (Unit: ppm)    B        P      Fe      Al   Ti   Ca    C     O    ______________________________________    Metallic           7     25     1010  800  180  950   ˜5000                                                    --    silicon    Ingot  7     23     10    25   3    13    6     40    after    smelting    in crucible    Wafer  0.1   <0.1   <0.1  <0.1 <0.1 <0.1  4      1    ______________________________________

In conclusion, the advantages of the manufacturing process and apparatusof polycrystalline silicon and manufacturing process of polycrystallinesilicon wafers for solar cells according to the present invention willbe summarized below compared with the conventional ones.

The processes for manufacturing polycrystalline silicon andpolycrystalline silicon wafers for solar cells according to the presentinvention are free from the source-wise problem (in other words,shortage in raw materials does not occur), do not by-produce pollutantsand are essentially suited to the scale up of the equipment and massproduction because of a metallurgical technique employed. It istherefore possible to supply wafers stably even if the demand for solarcells will increase by several hundred times in future. In addition,during the manufacture of wafers from high-purity silicon in the massform, about 20 wt. % of losses and inferior products appear as a resultof pulverization or the like. Continuous and consistent manufacture fromsilicon to wafers according to the present invention, on the other hand,reduces losses, whereby electricity and energy can be used effectively.The price of the silicon wafer available in the enforcement of thepresent invention can be reduced to half of that of the conventionalproduct, which makes it possible to allow the solar cell to functioneconomically as an electricity generating apparatus.

What is claimed is:
 1. A process for manufacturing polycrystallinesilicon from metallic silicon, comprising the steps of:melting metallicsilicon under a vacuum to remove the phosphorus contained in saidmetallic silicon by evaporation, thereby generating a residue containingimpurity elements; carrying out directional solidification bysolidifying said residue upwardly from a lower portion of said residueto an upper portion of said residue thereby obtaining a first ingot,wherein said first ingot contains an impurity concentrated portion;physically removing said impurity concentrated portion of said firstingot, leaving a remaining portion of said first ingot having a reducedimpurity concentration; re-melting said remaining portion of said firstingot, thereby forming a melt; removing boron and carbon from said meltby oxidizing said melt in an oxidizing atmosphere, and in succession,blowing argon gas or a mixed gas of argon and hydrogen into said meltfor deoxidization thereby forming a deoxidized melt; casting saiddeoxidized melt in a mold; subjecting the resulting deoxidized castingto directional solidification wherein said deoxidized casting issolidified upwardly from a lower portion of said deoxidized casting toproduce a second ingot wherein said second ingot contains a secondimpurity concentration portion; and physically removing said secondimpurity concentrated portion of said second ingot to obtain thepolycrystalline silicon.
 2. A process according to claim 1, wherein saidmetallic silicon is obtained by reductive smelting of silicon oxide. 3.A process for the production of polycrystalline silicon, whichcomprises:transferring a melt of metallic silicon, obtained by reductivesmelting of silicon oxide, into a crucible, removing boron and carbon byoxidizing said melt in an oxidizing atmosphere, subjecting said melt todirectional solidification to obtain a first ingot, said first ingothaving a first impurity concentrated portion, said directionalsolidification comprising solidifying said molten metallic siliconupwardly from a lower portion of said melt to an upper portion of saidmelt; physically removing said first impurity concentrated portion ofsaid first ingot, leaving a remaining portion of said first ingot havinga reduced impurity concentration; melting said remaining portion of saidfirst ingot under a vacuum thereby forming a re-melt and removingphosphorus from said re-melt; casting said re-melt in a mold, forming are-melt casting; subjecting said re-melt casting to directionalsolidification wherein said re-melt casting is solidified upwardly froma lower portion of said re-melt casting to produce a second ingotwherein said second ingot contains a second impurity concentratedportion; and physically removing said second impurity concentratedportion of said second ingot to obtain the polycrystalline silicon. 4.The process according to claim 1 or 3, wherein said oxidizing atmosphereis formed from a gas selected from the group consisting of H₂ O, CO₂,and O₂ gas, wherein the amount of said gas is controlled such that theinterface between said melt and said gas is covered with silicon oxide.5. The process according to claim 4, wherein said silicon oxide isremoved by locally heating said silicon oxide by plasma arc.
 6. Theprocess according to claim 1 or 3, wherein a gas is blown into the meltand said gas being selected from the group consisting of H₂ O, CO₂, andO₂ gas.
 7. The process according to claim 1 or 3, wherein a moldreleasing agent is used, said mold releasing agent being selected fromthe group consisting of SiO₂ and Si₃ N₄.
 8. The process according toclaim 1 or 3, wherein a solidification interface moving rate is set at 5mm/min or less for obtaining the first ingot, and a solidificationinterface moving rate for obtaining the second ingot is set at 2 mm/minor less.
 9. The process according to claim 1 or 3, wherein said firstingot is cut such that the impurity concentrated portion of about 30% orless of the height of said first ingot is removed.
 10. The processaccording to claim 1 or 3, wherein said phosphorus concentration of thepolycrystalline silicon is 0.3 ppm or less.
 11. The process according toclaim 1 or 3, wherein said boron concentration of the polycrystallinesilicon is 0.6 ppm or less.
 12. The process according to of claim 1 or3, wherein said carbon concentration of the polycrystalline silicon is10 ppm or less.
 13. An apparatus for manufacturing polycrystallinesilicon, which comprises a retaining container for retaining metallicsilicon, heating means for heating said retaining container, therebymaintaining said metallic silicon in a molten state; a first mold intowhich said metallic silicon in said molten state is cast from saidretaining container; a vacuum chamber for removing phosphorus from saidmetallic silicon by evaporation, wherein said vacuum chamber surroundingsaid retaining container and said first mold; means for effectingdirectional solidification of cast metallic silicon, wherein a firstcast silicon ingot is formed, said first cast silicon ingot having afirst impurity concentrated portion; means for removing said firstimpurity concentrated portion of said first cast silicon ingot, leavinga remaining portion of said first cast silicon ingot; a smeltingcontainer positioned to receive said remaining portion of said firstcast silicon ingot, said smelting container having a re-melting meansfor controllably heating at least a portion of said remaining portion ofsaid first cast silicon ingot, thereby forming a re-melt, said smeltingcontainer having a spraying means for spraying a gas from a gas sourceon said re-melt, said gas source comprising a gas selected from thegroup consisting of an oxidizing gas, a hydrogen gas and a mixed gas ofhydrogen and argon, and a second mold positioned to receive saidre-melt, means for effecting directional solidification of said re-meltfor forming said re-melt into a second cast silicon ingot having asecond impurity concentrated portion; means for removing said secondimpurity concentrated portion of said second cast silicon ingot.
 14. Anapparatus according to claim 13, wherein the degree of vacuum in saidvacuum chamber is set at about 10⁻³ torr or higher.
 15. An apparatusaccording to claim 13, wherein said retaining container is a watercooled copper crucible or a graphite crucible; and the smeltingcontainer is selected from the group consisting of a crucible made ofSiO₂, SiO₂ stamped crucible, and a SiO₂ lined crucible.
 16. An apparatusaccording to claim 13, wherein said heating means is an electron gun.17. An apparatus according to claim 13, wherein said re-melting means isselected from the group consisting of a plasma torch and a DC arcsource.
 18. An apparatus according to claim 13, wherein said first andsecond molds have side walls formed of a heat insulating material andhave a bottom formed of a water-cooled jacket and wherein a heatingsource for heating the melt is disposed above the casts.
 19. Anapparatus according to claim 13, wherein said first cast silicon ingotand said second cast silicon ingot have a diameter W and a height H, andthe ratio of W/H is set at 0.5 or more.
 20. A process for the productionof a silicon wafer for a solar cell, which comprises slicing an ingot ofpolycrystalline silicon obtained in the process according to claim 1 or3 into thin plates having a thickness of about 100 to 450 μm.